Inkjet printing is a non-impact method for producing images by the deposition of ink droplets in a pixel-by-pixel manner to an image-recording element in response to digital signals. There are various methods that may be utilized to control the deposition of ink droplets on the image-recording element to yield the desired image. In one process, known as continuous inkjet (CIJ), a continuous stream of droplets is charged and deflected in an image-wise manner onto the surface of the image-recording element, while un-imaged droplets are caught and returned to an ink sump. In another process, known as drop-on-demand inkjet, individual ink droplets are projected as needed onto the image-recording element to form the desired image. Common methods of controlling the projection of ink droplets in drop-on-demand (DOD) printing include piezoelectric transducers and thermal bubble formation (thermal inkjet (TIJ)). Inkjet printers have found broad applications across markets ranging from industrial labeling to short run printing to desktop document and pictorial imaging.
The inks used in the various inkjet printers can be classified as either dye-based or pigment-based. A dye is a colorant that is dissolved in the carrier medium. A pigment is a colorant that is insoluble in the carrier medium, but is dispersed or suspended in the form of small particles, often stabilized against flocculation and settling by the use of dispersing agents. The carrier medium can be a liquid or a solid at room temperature in both cases. Commonly used carrier media include water, mixtures of water and organic solvents and high boiling organic solvents, such as hydrocarbons, esters, ketones, etc. Aqueous-based ink compositions are preferred because they are more environmentally friendly as compared to solvent-based inks, plus most printheads are designed for use with aqueous-based inks.
One disadvantage to the use of dyes is their poor image stability, especially with respect to light and ozone induced fade mechanisms. Different methods are disclosed to improve the ozone stability of inkjet images. U.S. Pat. Nos. 6,854,840 and 6,087,051, e.g., teach the use of an overcoat to form a barrier to ozone such that the ozone stability of the prints can be improved.
It is also known that ion-exchangeable clays can be used to fix water-soluble dyes. U.S. Pat. No. 5,543,453, e.g., relates to the use of clay in an overcoat for thermal transfer dye fixing by ionic exchange. U.S. Pat. Nos. 5,560,996 and 6,281,270 relate to the use of ionic clays having ionic exchange capability for ionic dyes wherein the clay is put into or coated onto the printing paper prior to printing with ionic dyes that intercalate into the clay by ion exchange. U.S. Pat. Nos. 5,644,350, 6,063,836, and 6,177,501 relate to the use of ion-exchangeable clays in paper as well. U.S. Pat. No. 6,033,466, relates to the making of a pigment from the interaction of ionic dyes and intercalatable clays, wherein the ionic dyes are intercalated into the clay by ion exchange forming a precipitate and the precipitate is milled and redispersed into a “pigment” ink. In each of such dye intercalated clay teachings, the charge on the main surfaces of the clay layers and on the dye has an opposite sign. Forming stable printing ink compositions comprising intercalated clay-dye pigment compositions has been found to be problematic, as the formed pigment compositions frequently require milling to achieve a desired colorant particle size, and farther require use of additional dispersing agents to form a stable pigment dispersion.
It is an object of this invention to provide an ink for inkjet printing comprising a dye colorant, that improves the durability of the formed inkjet printed dye image. It is another object of this invention to provide such an ink for inkjet printing wherein the image quality of the printed image is improved.